Some conventional rotation angle detecting devices detect rotation angle of a detection object, which is a rotary member, without using magnetic flux collecting members such as stators. A rotation angle detecting device disclosed in JP 3491577, for example, has no magnetic flux collecting members so that its configuration is simplified and cost is reduced. This rotation angle detecting device according to JP 3491577 calculates an output voltage value, which corresponds to a relative rotation angle between a magnetism generating member and a magnetism sensing element, based on a magnetic force detection value (detection value of magnetic force), which is applied to and detected by the magnetism sensing element. Since this rotation angle detecting device has no magnetic flux collecting members, the magnetism sensing element outputs a detection voltage, which varies in a sine waveform or a cosine waveform. The magnetic force detection value varies non-linearly near a rotation angle, where the magnetic force detection value takes a maximum value. Accuracy in rotation angle detection is likely to be lowered particularly in a range including such a rotation angle. This rotation angle detecting device therefore converts the output voltage to a value, which varies linearly with the rotation angle, by calculating the output value of the magnetism sensing element based on an arcsine function or an arccosine function.
However, in this rotation angle detecting device, the magnetic force detection value of the magnetism sensing element varies when the magnetism generating member generates less magnetism or deviates in position. In this case, the output voltage is likely to vary largely near the rotation angle, which corresponds to the maximum value of the magnetic force detection value. If the output voltage varies largely, the accuracy of rotation angle detection is lowered.